#include <fstream>
#include <cmath>
#include <iomanip>
#include <vector>

int main()
{
   std::ifstream input("in.txt");
   std::ofstream output("out.txt");
   std::vector<double> x, y;
   double read_x, read_y;
   while (input >> read_x >> read_y)
   {
      x.push_back(read_x);
      y.push_back(read_y);
   }
   x.push_back(x[0]);
   y.push_back(y[0]);
   double sqr = 0;
   for (size_t i = 0, len = x.size() - 1; i < len; i++)
   {
      sqr += x[i] * y[i + 1] - x[i + 1] * y[i];
   }
   output << std::setprecision(9) << fabs(sqr)/2;
   return 0;
}